Conventional motion picture films incorporate a sound track having a single channel of sound recorded thereon. However, in order to provide for greater versatility, it is desirable to record two channels of sound on the sound track, in parallel channels. Thus two sound channels can be recorded in different languages. Stereo sound tracks, for use in 35 mm film in large theaters, may be recorded in the same way. A greater number of sound channels may give still further versatility so that three, four or five sound channels may be usefully employed in some cases. In the case of two sound channels the width of each channel of sound will be slightly less than half of the total width. Obviously, the signal to noise ratio will be reduced, and the intrusion of noise signals over the actual sound signals will be greatly increased. For these reasons, the use of dual sound tracks or channels on such motion picture films has been of only very limited application.
In addition the mechanical problems involved in sensing the optically recorded signals on each channel are greatly increased where more than one such channel is used, particularly on 16 mm film where the sound track is already narrow in itself.
For example, in the 16 mm motion picture film width, the space available for the sound track is only about 1.5 mm wide. In addition, it is necessary to leave a narrow margin or "septum" between the two sound tracks, usually about 0.5 mm, which still further restricts the space available for recorded sound. Thus if it is attempted to record dual sound tracks on two channels side by side, each sound channel will be about 0.5 mm.
Obviously, any sound recorded on such reduced width channel, which for example receives a scratch or abrasion, will be distorted to a proportionately greater degree, than if the sound were recorded on the entire width of the space available.
Conventional sound systems for single sound track film involve a light source and a light sensitive device which may be sensitive over a relatively wide area. This however is not harmful as it is a monaural sound track and there is no possibility of picking up interference from an adjacent channel.
Where multiple sound channels are used however the light sensitive device must be capable of reacting with maximum sensitivity to one channel without any interference or "cross-talk" from the others.
It is therefore desirable for all these reasons that the sensing devices for generating the sound signals in response to the sound recorded on the multiple channels be more accurate and sensitive. It is particularly important that, when using one of the sound channels on a selective basis, for example, where the sound channels are in two different languages, the sensing device be effective to pick up the signals from one sound channel, without receiving any interference or distortion from signals recorded on other sound channels.
In addition to the foregoing considerations, it is also of course desirable that the sensing device should be capable of being installed as a modification to existing equipment, as well as being installed as an optional feature in new equipment.
In addition to the foregoing considerations, considerable difficulties are experienced in adjusting to achieve optimum performance on both channels, when using a length of actual film with a multiple sound track. Accordingly, it is desirable to create a specially recorded calibration film having a sound track recorded thereon which is particularly adapted and suitable for the calibration of the sensing device, the calibration film strip having the same number of sound channels, for calibration purposes, as will be carried on the actual film to be projected.